AAOmega - Spiral

Acquisition notes and Overheads

The SPIRAL system uses the AAOmega spectrograph and so many of the principles of operation are identical to that of MOS observing with the 2dF tip end. However, a number of additional considerations must be addressed. The user new to IFU spectroscopy should discuss options with their support astronomer well in advance of the observing run.

Blind pointing
CASS acquisition system
Readily identifiable targets
Position Angle (PA)
Blind offsetting
IFU reconstruction
Finding charts
Guide stars
Mosaicking and acquisition offsets
CCD readout
Flats and arcs
Spatial and spectral scales

Blind pointing

The AAT pointing will not normally be sufficient for a blind pointing acquisition, even with the wide FoV of the IFU. The CASS acquisition camera system is the default acquisition mode.

CASS acquisition system

The A&G system at CASS has a 45degree mirror system which can be interchanged with the SPIRAL feed. With an up-to-date telescope pointing model (this is regularly re-derived for the telescope and can be performed in twilight when required) and an accurately defined instrument aperture offset (the offset between acquisition camera and IFU centers) a readily identifiable astronomical target can be acquired with the A&G camera, and accurately centered on the IFU within 5minutes at the <0.7arcsec single lens level, repeatably over a number of nights of observation.

Readily identifiable targets: For a clean acquisition of the target object using the CASS camera, the target must be easily identified in a short exposure. Finding charts are essential for all but the most trivial of targets. For extended source with no clear center acquisition accuracy is clearly compromised and for such targets the user should consider providing a nearby bright star which can be acquired and then an accurate guide probe offset can then be performed to the target. The camera runs a 1k Tek CCD and can readily observe stars to 18th mag in full moon, 20th in dark time. Brighter stars of course require less integration and suitable dark/sky frames would also not be need, so brighter guide stars will give a speedier setup. The star must be clearly identifiable, i.e. :

Not a binary
Well separated from bright neighboring stars, since the night assistant will not know which star to use.
Have a small, or well defined proper motion
Come with a finding chart when ever possible.

Position Angle (PA)

The SPIRAL IFU head unit can be freely orientated over a range in Position Angle (PA) of +/- 90 degrees (PA=0 is defined as the long axis running East-West). However, modifications to the PA require the telescope to be parked at Zenith and the head unit manually adjusted by AAT staff. Adjustments to the head unit will also require a redetermination of the instrument aperture.

Blind offsetting

Blind offsetting is a standard procedure for most telescope. When the target is ill defined in an acquisition image (it may, for example, be to faint for a short exposure, extended with no clear defining feature or perhaps an emission line source not easily seen in a broad band image) then the user will define a nearby bright (<18mag) acquisition star. The acquisition star is then acquired using the CASS camera system and the telescope guiding started. The user then supplies clearly defined offsets from the acquisition star to the well defined position in the target object. The guide probe is then moved to the correct position and the telescope offset so as to place the guide star at the offset guide probe position, hence the telescope is now tracking accuratly on a well defined target position.

This process requires:

A bright (<18mag) acquisition star
Well defined offsets

The night assistant must chose a telescope guide star that can be seen by the guide probe when observing the acquisition star AND the final target position. The user must therefore keep the offset as small as possible to move the guide probe as little as possible. In practice 1-10arcmins is usualy sufficient.

This process need not take any longer than a standard SPIRAL acquisition (~5min).

IFU reconstruction

The 2dfdr data reduction software allows a real time reconstruction of IFU observation, creating a 2D image by summing over wavelength. This process is used to define the instrument aperture and to check acquisition on bright stars in twilight. However, for science target acquisition, it is of little value as long science observations would be required for acquisition of faint targets. Direct CASS camera acquisition or Blind offsetting are almost always the correct method of target acquisition.

Finding charts

Finding charts need not be overly elaborate, however one must follow some basic rules:

Images should be approximately 30x30 arcmins
A printable PS/PDF file is the preferred format. FITS file are not recommended as they can be missinterpreted at the telescope.
The CASS camera works in approximately the V-band. It is not essential that the finding chart is given in this band, however the chart MUST resemble the V-band image to be of use (please, no IRAC 8micron image finders).
Black and white please, and with good contrast. If a colour figure is to be sent, please make a black and white copy as well and ensure it prints clearly.
Clearly mark the target
Clearly mark the acquisition star if one is to be used
Give the PA (if required)
An IFU overlay on the image can be useful for complex targets, but is not essential.
Give the image scale as a scale bar
Give the orientation
A long and complex text explanation accompanying a poorly constructed finding chart is of no value. A single A4 chart can and should accurately convey all of the relevant information succinctly.

The STARLINK Gaia, ESO skycat or ds9 image tools all produce adequate charts using DSS or similar images. The user may need to add adequate annotation to these images.

Guide stars

During SPIRAL observations the telescope tracking is controlled using the A&G unit guide probes. The system is operated by the night assistant and should usually be transparent to the astronomer. Some considerations require the user consult with the night assistant prior to the choice of guide star:

If a series of mosaicking offsets is to be performed, or a large blind offset from an acquisition star is required, a guide star must be chosen such that the star will remain visible to the guide probe throughout the operation.
The guide probe operates at a fixed wavelength. Since SPIRAL does not have an Atmospheric Dispersion Compensator (ADC) at this time, the TCS must take account for the differential tracking effect if one is to observed at a wavelength well away from that of the guide camera.

Mosaicking and acquisition offsets

The user needs to predetermine the relevant offsets from acquisition stars and for mosaicking dithered IFU frames. Offsets are applied by offsetting the guide probe in arcsecsons North-South and East-West. Size and direction of offset will need to be clearly communicated to the night assistant at the telescope.

Note the IFU has a number (3 - September 2007) dead fibres. If the user wishes to recover this lost in formation it is suggested that small, 0.7arcsec, integer lens offsets are applied to dither observations. Due to the method of IFU head unit construction and the fibre mapping onto the spectrograph slit unit, it is suggested that these dithers be in the short axis direction when possible (North-South at PA=0) as this will move the target between adjacent banks of fibre on the CCD rather than between fibres within an individual slit unit on the CCD (contact your support astronomer to discuss this in more detail).

If the PA is to be none zero, the user will need to work out the sense of these integer lens offsets on the sky prior to observation.

CCD readouts

The AAOmega CCD readout information can be found here. Most projects will use the NORMAL readout mode, although FAST is appropriate for some projects. The readout time in NORMAL mode is 2mins.

Flats and arcs

Located in the West Coude room on the 4th floor of the AAT, the AAOmega system is free from flexture. Environmental changes (temperature, pressure) will cause changes in the system on a day-to-day time scale. Current experiance with SPIRAL is limited but it is suggested that flat field and arc frames are not required more frequently than every few hours. Typicaly these calibration frames are taken at regualr intervals while slewing between targets.

Spatial and spectral scales

The lens scale of the SPIRAL IFU is 0.7arcsec square pixels.
The spectra project to a FWHM of ~2.4 pixels at the CCD.
The resolution of the SPIRAL system is x1.5 that of MOS mode due to the smaller fibres, altough the PSF is less well sampled (FWHM~2.4pixels)

Rob Sharp (rgs@aao.gov.au)